Gas turbine engines often use an air turbine starter to start the gas turbine. The air turbine starter is usually mechanically connected to the gas turbine engine, and receives compressed air from an air source such as an auxiliary power unit or another compressed air such as an air compressor.
The compressed air enters a manifold and is distributed to a number of nozzles or other apertures that direct the compressed air to the turbine starter turbine that rotates when the compressed air exits the nozzles and impinges on the turbine starter turbine. The turbine starter turbine is mechanically connected through shafts and gearing to the gas turbine engine, which is driven to rotate to a starting speed by the air turbine starter. When the gas turbine engine is rotating at a starting speed, the gas turbine engine may be started and the air turbine starter may be mechanically disengaged from the gas turbine engine.
Typically the compressed air that enters the manifold of the air turbine starter is controlled by a valve that controls the flow of compressed air to the air turbine starter. Thus, when the valve is open the compressed air flows to the air turbine starter, and the gas turbine engine core rotates.